I don’t know because I don’t edit YouTube releases, but about getting a good image, if you have yt-dlp command line tool – available on Linux, Macintosh, Windows – you can use:
yt-dlp --skip-download --write-thumbnail --convert-thumbnails png <YouTube video ID or URL>
Use png because CAA does not support WebP images yet.
Otherwise a web search like youtube thumbnail downloader should help. 